Nonaqueous electrolyte secondary battery anode material with a uniform metal-semiconductor alloy layer

ABSTRACT

The present invention relates to nonaqueous electrolyte secondary batteries and durable anode materials and anodes for use in nonaqueous electrolyte secondary batteries. The present invention also relates to methods for producing these anode materials. In the present invention, a metal-semiconductor alloy layer is formed on an anode material by contacting a portion of the anode material with a displacement solution. The displacement solution contains ions of the metal to be deposited and a dissolution component for dissolving a part of the semiconductor in the anode material. When the anode material is contacted with the displacement solution, the dissolution component dissolves a part of the semiconductor in the anode material thereby providing electrons to reduce the metal ions and deposit the metal on the anode material. After deposition, the anode material and metal are annealed to form a uniform metal-semiconductor alloy layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 12/105,090,filed Apr. 17, 2008, which is incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

The present invention relates to nonaqueous electrolyte secondarybatteries and relates specifically to more efficient and durable anodematerials for use in nonaqueous electrolyte secondary batteries as wellas methods for producing these anode materials.

BACKGROUND OF THE INVENTION

Nonaqueous electrolyte secondary batteries are a type of rechargeablebattery in which ions move between the anode and cathode through anonaqueous electrolyte. Nonaqueous electrolyte secondary batteriesinclude lithium-ion, sodium-ion, and potassium-ion batters as well asother battery types.

Lithium-ion batteries are a popular type of nonaqueous electrolytesecondary battery in which lithium ions move between the cathode and theanode thought the electrolyte. The benefits and the challenges oflithium-ion batteries are exemplary of the benefits and challenges ofother nonaqueous electrolyte secondary batteries; the following examplespertaining to lithium-ion batteries are illustrative and are notlimiting. In lithium-ion batteries, the lithium ions move from the anodeto the cathode during discharge and from the cathode to the anode whencharging. Lithium-ion batteries are highly desirable energy sources dueto their high energy density, high power, and long shelf life.Lithium-ion batteries are commonly used in consumer electronics and arecurrently one of the most popular types of battery for portableelectronics because they have high energy-to-weight ratios, no memoryeffect, and a slow loss of charge when not in use. Lithium-ion batteriesare growing in popularity for in a wide range of applications includingautomotive, military, and aerospace applications because of theseadvantages.

FIG. 1 is a cross section of a prior art lithium-ion battery. Thebattery 15 has a cathode current collector 10 on top of which a cathode11 is assembled. The cathode current collector 10 is covered by aseparator 12 over which an assembly of the anode current collector 13and the anode 14 is placed. The separator 12 is filled with anelectrolyte that can transport ions between the anode and the cathode.The current collectors 10, 13 are used to collect the electrical energygenerated by the battery 15 and connect it to an outside device so thatthe outside device can be electrically powered and to carry electricalenergy to the battery during recharging.

Anodes of nonaqueous electrolyte secondary batteries can be made fromcomposite or monolithic anode materials. In composite anodes,particulate anode material is physically bound together with a binderforming a matrix of the particles and the binder. For example, anodescan be made from carbonaceous particles bound with a polymer binder.Monolithic anodes are anodes that are not made by the addition of aphysical binder material. For example, any method of creating of asilicon anode where the silicon molecules are interconnected without theaid of an external binding agent is a monolithic film. Examples ofmonolithic anode materials include monocrystalline silicon,polycrystalline silicon and amorphous silicon. Monolithic anodes canalso be formed by melting or sintering particles of anode material or byvacuum and chemical deposition.

During the charging process of the lithium-ion battery, the lithiumleaves the cathode and travels through the electrolyte in the separatoras a lithium ion and into the anode. During the discharge process, thelithium ion leaves the anode material, travels through the electrolytein the separator and passes through to the cathode. Elements likealuminum, silicon, germanium and tin react with lithium ions and areused in high-capacity anodes. Anode materials that react with lithiumhave active areas in which lithium can react and inactive areas in whichlithium cannot react. The ratio of the active to inactive area of theanode affects the efficiency of the battery.

In the reaction of lithium ions in a lithium-reactive material, there isa significant volume difference between the reacted and extractedstates; the reacted state of lithium-reactive anode materials occupiessignificantly more volume than the extracted state. Therefore, the anodechanges volume by a significant fraction during every charge-dischargecycle. In lithium-reactive anodes, cracks in the anode material areoften formed during the cycling volume change. With repeated cycling,these cracks can propagate and cause parts of the anode material toseparate from the anode. The separation of portions of the anode fromcycling is known as exfoliation. Exfoliation causes a decrease in theamount of active anode material that is electrically connected to thecurrent collector of the battery, thereby causing capacity loss.

Silicon anodes, which are excellent candidates for lithium-ion batteriesdue to silicon's high capacity for lithium, suffer from significantcapacity degradation due to cycling exfoliation. Reducing thecharged-to-discharged voltage window applied to a silicon anode in alithium-ion battery can stem the capacity loss due to cycling since theexpansion and contraction are a function of the state of charge. Butreducing the charged-to-discharged voltage window lowers the operatingcapacity of the battery. Also, silicon is a poor conductor and mustoften be formulated with conductive additives to function as an anodematerial. These conductive additives reduce the active to inactiveratio, thereby reducing the energy density of the battery. Conductiveadditives are typically materials like carbon black that are added tothe anode particles and mixed before binding the particles.

Another method to improve conductivity of an anode material is todeposit a layer of conductive material on an anode material. Methods fordeposition of conductive layers include vapor deposition,electro-deposition, and electroless deposition. When materials aredeposited using any of the above methodologies on resistive substrateslike silicon, the deposition across the anode is typically non-uniform.For example, in electroless plating and electroplating of metals such asnickel, the deposition rate on a nickel surface is significantly higherthan that on a dissimilar surface such as silicon. A deposition that hassuch significant kinetic variations on different materials causes thedeposition to have surface defects, pores, and areas of no deposition.In the case of a line of sight deposition processes like vacuumdeposition from a target, non-planar surfaces with areas that are not indirect line of sight get significantly less or no deposition therebyreducing thickness uniformity. In addition, these coatings may notadhere well since these coating methods have poor adhesion strength ofthe deposited metal to semiconductor material. The poor adhesionstrength, poor uniformity, and poor minimum thickness of these coatingsresult in poor cycle life, power, energy, and reliability.

SUMMARY OF THE INVENTION

The present invention relates to nonaqueous electrolyte secondarybatteries and durable anode materials and anodes for use in nonaqueouselectrolyte secondary batteries. The present invention also relates tomethods for producing these anode materials. In the present invention, alayer of metal-semiconductor alloy is formed on an anode material bycontacting a portion of the anode material with a displacement solution.The displacement solution contains ions of the metal to be deposited anda dissolution component for dissolving a part of the semiconductor inthe anode material. When the anode material is contacted with thedisplacement solution, the dissolution component dissolves a part of thesemiconductor in the anode material thereby providing electrons toreduce the metal ions and deposit the metal on the anode material. Afterdeposition, the anode material and metal are annealed to form a uniformmetal-semiconductor alloy layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a prior art lithium-ion battery; and

FIG. 2 is an illustration of the displacement process of an exemplaryembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The inventors of the present invention have discovered that using adisplacement plating process to coat metal on a semiconductor-containinganode material followed by annealing results in thin and uniformmetal-semiconductor alloy coating for improved conductivity and reducedanode expansion without a substantial loss in efficiency.

The uniform metal-semiconductor alloy coatings of the present inventionprovide significantly better electrical conductivity than the nativesemiconductor structure of the anode itself. For example, typicalsilicon powder has a resistivity of 1-100 Ω/cm; whereas a nickelsilicide layer of composition NiSi has a resistivity of 10-60 μΩ/cm.This provides an enormous advantage for use in a nonaqueous electrolytesecondary battery, such as a lithium-ion battery, since the coating alsoreduces the amount of conductive additive needed to make a workinganode. Since nickel silicide by itself lithiates reversibly, acombination of pure nickel silicide on silicon constitutes an anodematerial with both excellent electrical conductivity and excellentlithium cycling ability. The addition of fewer additives in theelectrode also improves the specific energy density of the electrodesince it has a lower amount of inactive material in the electrode.

The uniform metal-semiconductor alloy coatings of the present inventionalso provide uniformly high electrical conductivity thereby improvingthe ability to lithiate the anode material surface uniformly, which inturn causes uniform expansion of the anode material and subsequentlyless capacity degradation due to exfoliation. Metal deposition andmetal-semiconductor alloy formation according to embodiments of thepresent invention are not dependant on the crystal orientation of theanode material.

For anodes in nonaqueous electrolyte secondary batteries, the presentmethod is superior to other methods of making metal-semiconductor alloycoatings due to its selectivity, uniformity, and the improved electricalconductivity of the resultant material. These beneficial propertiesresult in anode materials for nonaqueous electrolyte secondary batteriesthat provide longer cycle life, more uniform charge/discharge, higherrate capability, and higher specific energy density than currently usedanode materials.

The present invention is directed to a method for forming an anodematerial having a metal-semiconductor alloy layer. The method comprisesobtaining an anode material comprising a semiconductor; preparing adisplacement solution comprising ions of a metal and a dissolutioncomponent for dissolving part of the semiconductor in the anodematerial; contacting a portion of the anode material with thedisplacement solution, wherein the portion of the anode materialcontains the semiconductor; dissolving a part of the semiconductor fromthe portion of anode material; reducing the ions of the metal to themetal by electrons provided by the dissolution of the semiconductor;depositing the metal on the portion of the anode material; and annealingthe portion of the anode material and the deposited metal to form ametal-semiconductor alloy layer.

In a method of the present invention, an anode material having a uniformmetal-semiconductor alloy layer is formed. An anode material comprisinga semiconductor is prepared and a displacement solution comprising ionsof a metal and a dissolution component for dissolving semiconductormaterial in the anode material is prepared. Then, a portion of the anodematerial that contains the semiconductor is contacted with thedisplacement solution. Some of the semiconductor from the portion ofanode material contacted with the displacement solution dissolves in thedisplacement solution. The dissolution of the semiconductor reduces ionsof the metal in the displacement solution to the metal. The metaldeposits out of the solution onto the portion of the anode material. Theportion of the anode material and the deposited metal are then annealedto convert the deposited metal to metal-semiconductor alloy.

The specific example of nickel and silicon is used in the followingembodiment. However, the processes of the present invention can be usedto deposit many other metals such as other base metals like copper andcobalt or noble metals like silver, gold, platinum, palladium, orrhodium. Preferably the displacement solution contains ions of primarilyone metal, but the displacement solution can contain ions of multiplemetals. The anode material can contain semiconductors such as silicon orgermanium or alloys of semiconductor materials such as silicon orgermanium alloys with tin, zinc and manganese. The semiconductor in theanode material can be a compound semiconductor like a III-V compoundsuch as aluminum antimonide (AlSb), indium antimonide (InSb), galliumarsenide (GaAs), and indium phosphide (InP); or a II-VI compound such ascadmium telluride (CdTe), and cadmium-selenide (CdSe).

In the example of nickel deposition on a silicon-containing anodematerial, the nickel ions can be supplied to the displacement solutionby adding a nickel-containing salt, such as nickel sulfate or nickelchloride and the dissolution component can be a hydrolyzed fluoride ion,such as ammonium fluoride or hydrofluoric acid. The base material, inthis case silicon, dissolves in the solution providing the electrons forthe deposition of the nickel. Silicon at the surface of the anodematerial dissolves into solution. Nickel ions reduce on the surface ofthe silicon-containing anode material to deposit a metallic nickel filmthat is subsequently converted into nickel silicide by annealing. Sincethis process requires the dissolution of silicon for the deposition ofnickel, the deposition occurs only on the sites where the silicon candissolve. As a result, the film that is deposited on the silicon surfaceis extremely uniform, unlike in an electroless or electro-depositionprocess. Since it also involves the displacement of silicon atoms forthe deposition of a nickel atom, the adhesion of the metal coating onsilicon through this process is superior to an electroless depositionprocess.

Contact between the displacement solution and a portion of the anodematerial can be accomplished by various means including partial andcomplete immersion, coating and spraying. FIG. 2 is a schematic of adisplacement process in which the anode material 21 is immersed in acontainer 22 that contains the displacement solution 23. As shown, theanode material 21 can be in particle form. The anode material 21 canalso be monolithic and have a uniform or non-uniform shape and can bemade of multiple parts. In FIG. 2, the anode material is submerged inthe displacement solution 23. However, a portion of the anode material21 may be contacted with the displacement solution 23. The displacementsolution 23 contains the required amount of the salt of the metal to bedeposited along with an appropriate concentration of the dissolutioncomponent. The solution 23 can be kept within a set temperature rangefor a specified length of time in the container 22 for the metaldisplacement to occur. When the desired thickness of the metal isachieved, the anode material is removed from the solution to be rinsed,dried, and annealed.

In one example of this process, the average thickness of metal depositedby this method on anode materials can be between about 100 nanometersand 3 micrometers. In an example of this process, metal layers depositedon anode materials by this process can have thickness non-uniformitiesof less than about 25%. Thickness non-uniformity is defined as thequantity of the maximum thickness minus the minimum thickness, dividedby the average thickness. The term about as used herein means plus orminus 15% of the specified value. The above steps of the depositionprocess can also be repeated in cases where thicker metal-semiconductoralloy layers are necessary.

Annealing can be performed in an inert atmosphere to form a uniformmetal-semiconductor alloy coating. For example, if the annealingatmosphere in the nickel-on-silicon example contains too much oxygen,the deposited nickel can be converted to nickel oxide instead of nickelsilicide in the annealing step. Annealing conditions for obtaining anickel silicide layer of good conductivity and good alloy uniformity aredisclosed in Waidmann et. Al., Microelectronic Engineering; Volume 83,Issue 11-12, Pages 2282-2286. In one example, annealing can be doneusing a rapid thermal anneal process at temperatures of about 500° C.Different annealing conditions provide silicides of varyingconductivities due to the formation of different alloy compositions. Theannealing conditions can be tailored for the resultant specificconductivities by changing the temperature and time of anneal.

In certain cases, such as when the annealing cycle needs to be short dueto process time constraints, the annealing conditions can be designedsuch that not all the deposited metal gets converted to ametal-semiconductor alloy. Excess metal can impede lithiation of themetal-semiconductor alloy and semiconductor material. In such ascenario, the excess metal that is not converted to metal-semiconductoralloy can be etched off using a solution that selectively etches themetal without etching the metal-semiconductor alloy. For example, asolution of sulfuric acid in hydrogen peroxide can selectively etchnickel without etching nickel silicide.

In one embodiment of the method of the present invention, thedisplacement solution contains about 0.02-0.5 M Ni²⁺ as nickel sulfatein solution; contains about 0.5-5 M fluoride as NH₄F in solution; has apH of about 8-10; and has a temperature of about at 50° C.-98° C.

In another embodiment of the method of the present invention, thedisplacement solution contains about 0.02-0.5 M Ni²⁺ as nickel sulfatein solution; contains about 0.5-5 M peroxide as H₂O₂ in solution; has apH of about 8-10; and has a temperature of about at 50° C.-98° C.

The methods of the present invention can be used to prepare an anodematerial having a uniform metal-semiconductor alloy layer. Themetal-semiconductor alloy layer on an anode prepared by the methods ofthis claim is preferably a nickel silicide. Other metals may also beused on anodes prepared by the methods of this invention including otherbase metals like copper and cobalt or noble metals like silver, gold,platinum, palladium, or rhodium. Preferably, the thickness of themetal-semiconductor alloy on an anode prepared by the methods of thepresent invention is between about 100 nanometers and 3 micrometers.Preferably, the thickness non-uniformity of the metal-semiconductoralloy on an anode prepared by the methods of the present invention isless than about 25%.

A nonaqueous electrolyte secondary battery can be made using an anodeprepared by the methods of the present invention. A battery is formedusing the anodes of the present invention by combining them withcathodes and electrolytes in planar or three dimensional structures asknown in the art. See, e.g., Long et. al., Three-Dimensional BatteryArchitectures, Chemical Reviews, 2004, 104, 4463-4492. The nonaqueouselectrolyte secondary battery of the present invention can be alithium-ion battery, a sodium-ion battery, a potassium-ion battery, oranother type of nonaqueous electrolyte secondary battery.

The following examples further illustrate the present invention. Theseexamples are intended merely to be illustrative of the present inventionand are not to be construed as being limiting.

EXAMPLES Example 1 Immersion Nickel Deposition on Particulate AnodeMaterial

A sample of 2 grams of silicon particulates in powder form (−325 mesh)was immersed for 30 seconds in 50 milliliters of a solution containing0.1 M NiSO₄.6H₂O and 5M NH₄F. The pH of the solution was maintained at8.5 and the operating temperature was 85° C. Deposition was done withthe powder on top of a filter paper assembled in a Buchner funnel.Vigorous bubbles were observed during deposition indicating the nickeldisplacement reaction. The solution was drained out through theapplication of vacuum in the Buchner funnel. The sample was rinsed withDI water for 10 minutes to remove trace salt contamination. The powderwas harvested and dried at 80° C. in air for 12 hours. Subsequently, thepowder was annealed for 2 hours (including heat and cool time) to amaximum temperature of 550° C. in a H₂/N₂ atmosphere to form thesilicide.

The resulting anode material was reversibly cycled upwards of 1200 mAh/gof silicon equivalent for 100 cycles at an average coulombic efficiencyof 99.8% for the 100 cycles. This was in contrast to a silicon powderalone which degraded to less than 330 mAh/g capacity of silicon by the10th charge-discharge cycle.

Example 2 Immersion Nickel Deposition on Particulate Anode Material

The process of Example 1 was carried out on particulate silicon inpowder form (−325 mesh). The sample prepared was immersed in a solutionof 10 ml H₂SO₄+40 milliliters 3% H₂O₂ at 70° C. for 3 minutes to removeany excess nickel and to leave behind the silicide.

Although the invention has been described with reference to thepresently preferred embodiments, it should be understood that variousmodifications could be made without departing from the scope of theinvention.

What is claimed is:
 1. A composite anode for a secondary battery, thecomposite anode comprising a matrix comprising a binder and aparticulate anode material bound together by the binder, the particulateanode material comprising a semiconductor material having a surface anda semiconductor alloy layer completely coating the semiconductormaterial surface, the semiconductor material comprising silicon,germanium, a III-V compound semiconductor or a II-VI compoundsemiconductor, the semiconductor alloy layer comprising an alloy of thesemiconductor material and a metal selected from nickel, copper, cobalt,silver, gold, platinum, palladium and rhodium, the semiconductor alloylayer having a thickness of about 100 nanometers to about 3 micrometersand a thickness non-uniformity of less than about 25% wherein thicknessnon-uniformity is defined as the quantity of the maximum thickness minusthe minimum thickness, divided by the average thickness.
 2. Thecomposite anode of claim 1, wherein the metal is a noble metal.
 3. Thecomposite anode of claim 1, wherein the metal is a base metal.
 4. Thecomposite anode of claim 3, wherein the metal is nickel.
 5. Thecomposite anode of claim 1, wherein the semiconductor is silicon.
 6. Thecomposite anode of claim 1, wherein the metal is nickel and thesemiconductor is silicon.
 7. The composite anode of claim 1, comprisinga metal-semiconductor layer wherein the average thickness of themetal-semiconductor alloy layer is between about 100 nanometers and 3micrometers.
 8. The composite anode of claim 1, comprising ametal-semiconductor layer wherein the thickness non-uniformity of themetal-semiconductor alloy layer is less than about 25%.
 9. The compositeanode of claim 1 wherein the semiconductor material contains silicon andthe semiconductor alloy layer comprises an alloy of nickel or copper.10. The composite anode of claim 9 wherein the semiconductor materialcontains silicon and the semiconductor alloy layer is a nickel silicidelayer having a resistivity of 10-60 pa/cm.
 11. The composite anode ofclaim 9 wherein the semiconductor alloy layer is a nickel silicide layerand the nickel silicide layer is formed by a process comprising the stepof selectively etching the nickel silicide layer to remove excess nickelwithout etching the nickel silicide layer.